Because the eyes require to be _defended_ from floating particles in the air, and to be kept _moist and clean_. The eyelids form the shutters of the eye, defending it when waking, by closing upon its surface whenever danger is apprehended, moistening its surface when it becomes dry, and covering it securely during the hours of sleep. 973. _Why are the eyelids fringed with eyelashes?_ Because the eyelashes assist to modify the light, and to protect the eye, without actually closing the eye-ids. When the eyelids are partially closed, as in very sunny or dusty weather, the eyelashes cross each other, forming a kind of shady lattice-work, from the interspaces of which the eye looks out with advantage, and sees sufficiently for the guidance of the body. 974. _Why are we able to see at long or short distances?_ Because the _crystalline lens_ of the eye is a moveable body, and is pushed forward, or drawn back by fine muscular fibres, according to the distances of the objects upon which we look. By these means its _focus_ becomes adjusted. 975. _Why do we wink?_ Because, by the repeated action of winking, _the eye is kept moist and clean_, and the watery fluid secreted by little glands in the eyelids, and at the sides of the eye, is spread equally over the surface, instead of being allowed to accumulate. But the action of winking, or brightening the eye, is so instantaneous that it does not impede the sight. [Verse: "And the eye cannot say unto the hand, I have no need of thee; nor again the head to the feet, I have no need of you."--CORINTH. XII.] 976. _Whence are the humours and secretions of the eye derived?_ From the blood, which flows abundantly to the eyes, and is circulated in capillary vessels that are spread out upon the membranous coats of the eye-balls. [Illustration: Fig. 63.--SECTION OF THE EYE.] A and B. The _sclerotic_, _choroid_, and _retina_, the three layers or coats which form the walls of the globe of the eye, and enclose its humours. C C. The _iris_. D. The front chamber of the eye, filled with watery humour. E. The _pupil_, through which the rays of light pass to F. The _crystalline lens_. G G. The _vitreous humour_ enclosed in cells formed by the _hyaloid membrane_. H. An _artery_ which supplies blood to the _crystalline lens_, and which passes through the centre of the _optic nerve_. G. The _optic nerve_, showing the sheath in which the nerve is enclosed. 977. _Why do tears form in the eyes?_ Because, under the emotions of the mind, the circulation of blood in the brain, and in its nearest branches, becomes considerably quickened. The eyes receive a larger amount of blood, and the secretion of the lachrymal glands being increased, the fluid overflows, and tears are formed. The use of tears is probably _to keep the eyes cool during the excitement of the brain_. They are formed also during _laughing_, but less frequently.